It is known that sea water and brackish water can have the dissolved solids removed therefrom, that is, they can be desalinated, by the method known as reverse osmosis. The water is pumped, at a pressure of twelve to seventy Bar, through a desalination cartridge which uses a complex polymer as a semi-permeable membrane. The pressures used are required to overcome the natural osmotic pressure of the feed water. Pressures at the lower end of the range are used for brackish water and the higher pressures are used for sea water.
The type of cartridge most widely used in desalination comprises a multitude of fine hollow filaments of the complex polymer, the water being pumped into the spaces between the filaments. The filaments are of about the same cross sectional dimension as a human hair. Permeate (product water) flows through the walls of the filaments into their bores. The bores, which are also referred to as lumens, form the permeate passages. In this form of cartridge there are salt retention passages externally of the filaments and, as stated, the permeate water passages are constituted by the bores of the filaments.
The second type of cartridge which is widely used is the spirally wound type. In this form of cartridge flat sheets of the complex polymer are spirally wound onto a central core which is in the form of a hollow tube with a multitude of holes in it.
Between each adjacent pair of sheets of complex polymer there is a grid. The grids act as spacers which hold the sheets apart and form alternate salt retention passages and permeate passages between the sheets. The grids in the salt retention passages are intended additionally to introduce turbulence into the water flow. A stack of, for example, eight to twelve such sheets with spacers between them are wound onto the core simultaneously. The permeate passages spiral inwardly towards the core.
Older desalination plants tend to use cartridges comprising filaments. More recently installed plants use spirally wound cartridges. At present the total number of spirally wound cartridges in use is less than the number of cartridges using filaments. The disparity in numbers is, however, closing as most recently installed desalination plants use spirally wound cartridges and this is expected to continue in future.
A major problem with desalination is that of fouling of the semi-permeable membrane. Fouling arises from three sources. A major source is the deposition onto those surfaces of the polymer sheets or polymer filaments which bound the salt retention passages of layers of common salt and other solids such as magnesium and calcium. The sparingly soluble salts and other solids precipitate out as the water flows through the semi-permeable membrane from the salt retention passages to the permeate passages. This flow of water increases the concentration of the dissolved solids to such an extent that the remaining water is insufficient to maintain all the solids in solution.
The second source of fouling is organic. For example, algae, bacteria and the like grow on the semi-permeable membranes. The third source of fouling is solids which the filters conventionally used ahead of the desalination cartridge to remove solid particles fail to remove from the feed water.
Fouling is promoted by the fact that the polymer has a net negative charge whereas algae and bacteria have a net positive charge. Thus these organisms are preferentially attracted to the surfaces of the membrane where they are deposited and form colonies. Likewise, positive ions (cations) which are in sea water as a result of the disassociation of the dissolved solids are also preferentially attracted to the membrane.
It is well known that the rate of fouling of a cartridge increases in a non-linear manner with the permeate flow rate. Thus doubling the flow rate more than doubles the rate at which the desalination cartridge fouls. Fouling slows down the rate at which water flows through the membrane. Eventually the permeation rate decreases to such an extent that the cartridge must be defouled by treating chemically. A cartridge which has been excessively fouled may not, even after defouling, produce the same flow rate as it did prior to fouling.
As mentioned above, to minimize fouling of a conventional desalination cartridge including polymer sheets, turbulence inducing spacers are provided in the salt retention passages. Such spacers, by enhancing mixing of the water flowing in the salt retention passages, inhibit build-up of what is called the concentration polarization layer. This layer is immediately adjacent the membrane and in it the concentration of the dissolved solids is at a maximum. This layer not only forms a barrier to flow of water through the membrane but it is mainly from this layer that dissolved solids precipitate out to foul the membrane. Furthermore the existence of this layer of high concentration increases the osmotic pressure. Such turbulence inducing spacers only provide a partial solution to the fouling problem.
Fouling is a significant problem with desalination cartridges which include filaments as there are very fine interstices between the filaments in the mass of filaments. Hence the mass of filaments itself acts as an extremely efficient filter and intercepts and separates out any solid material in the feed water. Such material is retained in the mass of filaments and contributes to a reduction in the flow rate of the permeate.
In our PCT specification W097/21630 we propose a structure which introduces turbulence into the feed water entering the salt retention passages for the purpose of further inhibiting the build up of concentration polarization layers with consequent fouling.
The main object of the present invention is to enhance the performance of a desalination cartridge.
A further object of the present invention is to inhibit the rate at which the desalination cartridge fouls thereby permitting a greater product flow rate to be achieved over a longer period of time.